1. Field of the Invention
This invention relates to a method for excavation from the top down, usually known as "undercut" excavation which also comprises protection against seismic events such as rock bursts or earth quakes as well as from excessive but relatively slow ground movement. More particularly the invention relates to an undercut excavation method using concrete posts which are adapted to support concrete floors that become a roof for the next lower level of excavation and wherein the posts are combined with resilient elements to provide protection against seismic events or against ground movement that exceeds failure load of the concrete posts.
2. Discussion of the Prior Art
Applicant's U.S. Pat. No. 5,522,676 of Jun. 4, 1996 discloses an undercut excavation method wherein, as the first step, posts are inserted into the ground, which may be done by drilling holes in the ground and inserting concrete posts in such holes, and further these posts have top ends capable of supporting a concrete roof and are inserted into the ground so that their top ends are essentially flush with the ground; then a concrete floor is poured on the ground and on the top ends of the posts; and finally safe excavation proceeds beneath the concrete floor which now serves as a concrete roof for the excavation.
The above method also provides for a multi-level undercut excavation, such as an undercut-and-fill mining method, whereby the same procedure is repeated at each level as the excavation progresses downwardly from level to level until a desired number of levels has thus been excavated. In the undercut-and-fill mining method, the excavated rooms are back-filled with a suitable fill after excavating the same. Moreover, holes may be drilled around the posts inserted into the ground, and blasted with explosives to break the ground around the posts without, however, damaging the posts themselves. This facilitates excavation under the concrete floor/roof thereafter and minimizes damage to the posts during excavation.
It has also been disclosed in said U.S. Pat. No. 5,522,676 that additional posts may be stood-up in plumb on top of the posts previously inserted into the holes to provide further support to the concrete roof and thus an enhanced safety. This is called "double post" excavation, or when applied to mining "double post mining" or "DPM".
When a set of concrete posts is installed in holes in an undercut excavation as mentioned above or as part of the double post excavation or DPM, the posts have zero load. Once the concrete floor/roof has been cast and the excavation has been performed, there will be a load applied to the posts. If the excavation is only a one level excavation, it is likely that there may be a structure placed over it, such as a building or the like, which will exert an additional load onto the posts over and above the load exerted by the floor/roof poured thereover. The same applies to a multi-level excavation. Also in a mining undercut-and-fill method, loads are transmitted to the posts via the backfill as the rock or ore formations move or relax. The concrete posts are, of course, rigid and they could overload and fail particularly during seismic events, such as a rock burst or earth quake, which may produce massive energy releases.